A variety of catalyst compositions are known for the production of olefin polymers, including those based on traditional Ziegler-Natta catalysts. More recently, single site catalysts, compounds in which each catalyst composition contains one or only a few kinds of polymerization sites, have gained increased attention. Metallocenes are the most well known type of single site catalysts, and are organometallic coordination complexes containing one or more pi-bonded moieties (i.e., cycloalkadienyl groups) in association with a metal atom from Groups IIIB to VIII or the Lanthanide series of the Periodic Table of Elements. Single site catalysts are typically activated with aluminoxanes or non-coordinating anions when employed to polymerize olefins.
It is commonly believed among those skilled in olefin polymerization that catalyst compositions, particularly when used in heterogeneous polymerization methods such as gas phase polymerization and suspension polymerization, must be supported on an inert carrier to facilitate control of polymer particle size and bulk density. Even when mixed catalyst systems are utilized, both catalysts are supported on an inert carrier. For example, U.S. Pat. Nos. 4,701,432 and 5,077,255 to Welborn, Jr. relate to olefin polymerization catalyst compositions comprising a metallocene and a non-metallocene impregnated on a support. Similarly, EP 0 586 168 A1 discloses a catalyst composition comprising a metallocene complex having polymerizable groups and a Ziegler-Natta catalyst for the preparation of olefins. Preferably, the metallocene complex is in the form of a polymer and the catalyst composition is supported.
It has now been discovered that a particularly effective olefin polymerization catalyst comprises the combination of a heterogeneous, solid Ziegler-Natta catalyst and a homogeneous, liquid single site catalyst along with one or more activating cocatalysts. Polymers having broadened or bimodal molecular weight distributions are advantageously produced. Because the single site catalyst is introduced into the polymerization reactor as a liquid, adjustment of the Ziegler-Natta catalyst/single site catalyst feed ratio is easily accomplished. This in turn allows for better control of polymer molecular weight distribution and other properties. Moreover, the complexities and compatibility issues associated with supporting two catalysts on the same support are avoided. And in contrast to a mixed catalyst system in which the catalysts are supported on separate supports, with the present invention polymer made by each catalyst can grow on a single particle.